A casting or cast article is simply an article that is produced by allowing a molten metal to solidify in a mold. A cast article takes the shape of the mold in which it was cast. Cast articles are used as components in many industrial and consumer products.
Persons skilled in the art will understand, however, that this overly-simplified, generalized description of a casting is not intended by the inventors to mean that all processes that produce confined, solidified, once-molten metal are casting processes or that they produce castings. For example, persons skilled in the art recognize that a casting does not result from an infiltration process and that the process of infiltrating is not a casting process, even when the porous article that is being infiltrated is confined for support during infiltration; instead, they understand that what results from an infiltration process is an infiltrated article. Persons skilled in the art understand that infiltrating refers to a process in which a solidifiable liquid, such as a molten metal, is introduced into the interconnected porosity of a porous body by capillary force-driven or pressure-assisted wicking or imbibing. In other words, it is a process of filling the pores of a sintered or unsintered powder metallurgical compact with a liquid such as a molten metal which subsequently solidifies within the compact to form a dense article. Infiltration, by design and nature, is a complementary second step that is employed after a porous article has been produced. Its purpose is to densify the porous article while maintaining the overall structure of the porous article. In other words, the structure of the article is set in the process by which the porous article is made and then the density and other properties of the article are enhanced by the infiltration step.
Rather, a person skilled in the art would understand the term “casting” to refer to a process in which a solidifiable liquid, such as molten metal, is poured or injected into a mold and subsequently forms an article by solidifying in the shape of the mold. Such a person would understand casting to involve a solidifiable liquid flowing into a mold by bulk flow mechanisms and at volume flow rates that are substantially different from those predominating in infiltration. Note that whereas in infiltration the infiltrant fills voids and porosity within a preexisting porous article to densify the porous article, in conventional casting the article does not preexist. Rather, what preexists is a shaped volume defined by a mold and its adjacent free surfaces. During casting, the cast liquid is made to fill this shaped volume so that upon solidification of the liquid an article results having the shape of the shaped volume. Such an article is what one skilled in the art would call a “cast article” or more simply a “casting.”
The type of mold used depends in large part on the casting process selected. From the dawn of metal casting until recently, sand has been far and away the most common molding media into which the molten metal is cast. Molds may be made from other materials, for example, there are metal molds, graphite molds, and plaster molds. In many cases, patterns are used in the making of the mold in which the casting is formed. Some patterns may be re-used hundreds or even thousands of times, while others are single-use patterns which are destroyed during the mold making process.
There are a number of known casting processes, each with its particular benefits and drawbacks. These processes include sand casting, investment casting, gravity or low-pressure permanent mold casting, high-pressure die casting, thixomolding, centrifugal casting, plaster or shell mold casting, and squeeze casting.
Mold making is often a costly and time-consuming endeavor. The molds, or the patterns for the molds, may be machined to exacting detail by skilled craftsmen, sometimes using complex and expensive automated machining processes. The mold or pattern making requirements of casting processes often lead to long delivery times for the first cast articles. For example, investment casting may require three months to prepare the first casting. Die castings and permanent mold casting may require even longer lead times, which may approach six months. However, designers of new products increasingly require short turnaround times as they may change a design several times from initial ideal to final component.
Rapid prototyping may be used by designers to quickly obtain a three-dimensional model of a new design. The term “rapid prototyping” refers to a class of technologies that construct physical models from Computer-Aided Design (CAD) data in relatively short time periods. Rapid prototyping is also known within the art as “solid free form fabrication processing.” Rapid prototyping methods are sometimes referred to as “three dimensional printers,” because they allow designers to quickly create tangible three-dimensional prototypes of their designs from a computer file, rather than just two-dimensional pictures. The models produced by rapid prototyping have many uses. For example, they make excellent visual aids for communicating ideas with co-workers or customers. Additionally, wax models made by rapid prototyping methods have been used as patterns in the lost wax casting process. Although rapid prototyping methods are well suited to making prototypes, persons skilled in the art will understand that the products of rapid prototyping methods are not limited to simply being prototypes.
Two examples of commercially available rapid prototyping systems are three-dimensional printing (3DP) and selective laser sintering (SLS). Both of these processes build up a physical model on a layer-by-layer basis to produce a three-dimensional article made of powder bonded by a polymer binder. Such processes are capable of creating objects with complicated internal features, for example, passageways, that cannot be manufactured by other means. The 3DP process is conceptually similar to ink-jet printing. However, instead of ink, the 3DP process deposits a polymer glue. This polymer glue is printed onto a powder layer according to a two-dimensional slice of a three-dimensional computer representation of the desired object. The SLS process builds an article by fusing together polymer-coated powder particles. A computer-driven laser beam scans each powder layer and fuses together the polymer coatings of adjacent particles to bind the particles together in the form of a cohesive article.
The 3DP and SLS processes produce porous powder articles that typically consist of from about 30 to over 60 volume percent powder, depending on powder packing density and about 10 volume percent binder, with the remainder being void space. The porous powder article made by either of these processes is somewhat fragile and conventionally is thermally processed to yield a fully dense part having improved mechanical properties. A typical thermal process consists of debinding, powder sintering, and infiltrating the sintered article with a secondary molten metal.